The cranial cavity narrows at its cranial end to form the small =olfactory= fossa (Fig. 43, _III_) which lodges the olfactory bulbs. This is bounded by the frontals and the lamina cribrosa (Fig. 42, _o_) of the ethmoid; caudad it opens directly into the cerebral fossa. Numerous openings through the lamina cribrosa for the olfactory fibres connect the olfactory fossa with the nasal cavity. The roof of the fossa is marked by a prominent median crest from the united edges of the frontals. The =nasal= cavity is almost completely filled by the ethmoid and vomer and the conchæ nasales. Its roof is formed by the nasal bones and portions of the frontals; its sides by the frontals, lachrymals, maxillaries, premaxillaries, and palatine bones; its floor by the horizontal plates of the palatines, maxillaries, and premaxillaries. [Illustration: FIG. 43.--SKULL, MEDIAN LONGITUDINAL SECTION, SHOWING THE CAVITIES. _I_, cerebellar fossa; _II_, cerebral fossa; _III_, olfactory fossa. 1, occipital bone; 2, interparietal; 3, 3′, parietal; 4, temporal (4, squamous portion; 4′, petrous portion; 4″, tympanic portion); 5, sphenoid; 6, presphenoid; 7, palatine; 8, frontal; 9, maxillary; 10, premaxillary; 11, ethmoid; 12, nasal; 13, incisor teeth; 14, canine; 15, 16, 17, premolars; 18, molar. _a_, condyloid canal; _b_, hypoglossal canal; _c_, jugular foramen; _d_, internal auditory meatus; _e_, appendicular fossa; _f_, tentorium; _g_, dorsum sellæ; _h_, sella turcica; _i_, hamular process; _j_, pterygoid process of sphenoid; _k_, optic foramen; _l_, presphenoid sinus; _m_, _m′_, frontal sinus; _n_, lamina perpendicularis of the ethmoid (broken at cranial edge).]